Attention Deficit Hyperactivity Disorder (ADHD) is the most common behavioral diagnosis in childhood. It incurs high medical costs, and can contribute to poor academic achievement, adult mental illness, substance abuse, and criminal behavior. Standard treatments, such as stimulant medications, primarily target symptoms, and long-term follow-up studies of children treated for ADHD reveal that their outcomes remain significantly worse as compared to typically developing (TD) peers. Thus, there are tremendous potential public health benefits for behavioral training programs that could remediate core features of ADHD. To this end, it is worth considering the long-standing observation that children with ADHD often demonstrate difficulties with motor control, including motor impersistence and failure to inhibit motor overflow, that parallel (and correlate with) difficultie with measures of higher-order behavioral control. Further, recent functional imaging and transcranial magnetic stimulation (TMS) findings reveal children with ADHD show abnormal recruitment of cortical inhibitory mechanisms, and that these physiologic measures of motor disinhibition robustly correlate with parent ratings of children's ADHD symptoms. Given these findings, it follows that movement-based interventions that aim to achieve improved behavioral control through engagement of the motor system offer a promising approach for targeting specific biological substrates of ADHD. Gains in cognitive and behavioral control have been observed using mindful movement training, including the focus of this application: Tai Chi. Despite the markedly greater brain plasticity observed in children as compared to older adults, pediatric applications of mindful movement training has been under utilized and under investigated. Addressing this gap, we propose to examine the efficacy of a movement-based Tai Chi training for children with ADHD. Given the recognized need for physiologic biomarkers in mindfulness and movement training studies, we will track changes in specific motor behavioral and physiologic (from TMS) markers, in addition to assessment of core ADHD symptoms. Our proposed design employs a Fast Fail Model to establish feasibility for progression to a clinical trial of efficacy for Tai Chi, and potentially other forms of mindful movement training, for childrn with ADHD. In the R21 phase, children with ADHD will engage in an 8-week Tai Chi training, with pre- and post-training measurement of motor system function via TMS and behavioral testing. Qualitative measures of student and parent experience will also be collected to complement data on core motor system targets. Upon completion of project milestones, the R33 phase will incorporate measures of ADHD symptom severity, as well as Research Domain Criteria (RDoC) measures of cognitive control. Further, the R33 phase will include a mid-training (in addition to pre- and post-training) assessment and we will investigate the hypothesis that changes in motor system measures will precede changes in ADHD symptom severity. The proposed study offers immense potential for the development of novel therapeutic approaches for ADHD with little risk of adverse reaction.